Novel Coregulators of Estrogen Receptor in Enhancer-regulated Transcription Estrogen (E2 or 17b-estradiol) and its nuclear receptor ER? are critical for the normal development and disease conditions of multiple organs, including mammary glands. E2 and ER? regulate transcriptional programs in these biological contexts through binding primarily at distal enhancers. Increasing evidence indicates that functional dysregulation of ER?-bound enhancers profoundly alters normal transcriptional programs, leading to developmental defects, diseases, and hormone resistance. However, the molecular mechanisms underlying the enhancer function/dysfunction are largely unknown. My lab has been focusing on studying two key questions on ER?-bound enhancers: 1) how are estrogen-regulated enhancers assembled under different conditions? 2) how do the enhancer components encode the context-specific function of each individual enhancer in vivo? Using a powerful proximity proteomics approach, we recently identified additional ER?-interacting coregulators including YAP/TEAD, two key components of Hippo pathway to mediate nuclear effects. Our preliminary data suggest YAP/TEAD function in a non-canonical manner to interact with ER? on ER?-bound enhancers and play a key role in ER?-mediated transcriptional programs under normal signaling condition or during the development of hormone resistance . In this grant proposal, we are proposing two specific aims to test the hypothesis that YAP/TEAD are important novel coregulators required for the activation of ER?-bound enhancers, and the cooperative interactions between YAP/TEAD and ER? control the context-specific function of ER?-bound enhancers in vivo through enhancer reprogramming. The proposed work will provide a mechanistic interpretation on how YAP/TEAD and ER? signaling crosstalk at the chromatin level and converge on enhancers to control downstream gene expression under different conditions, and lay the foundation for future development of improved ER-targeted therapy.